1. Field of the Invention
This invention relates to an apparatus for recording and reproducing information on and from an optical disc. In addition, this invention relates to a method of recording and reproducing information on and from an optical disc. Furthermore, this invention relates to an optical disc.
2. Description of the Related Art
Optical discs contain an MD (Mini Disc). MD players include shock-proof memories having a capacity of 4 MB which corresponds to a playback time of about 10 seconds. During the playback mode of operation of the MD player, a pickup sequentially accesses sectors on an MD and reproduces data therefrom. In the MD player, the reproduced data are temporarily stored in the shock-proof memory and are read out therefrom so that the contents of the data are played back. When the pickup jumps from a sector to a next sector, the pickup does not reproduce any data from the MD. Thus, during the playback mode of operation of the MD player, the reproduction of data from the MD by the pickup is sometimes interrupted for a short time. The shock-proof memory absorbs such an interruption of the reproduction of data from the MD, thereby providing continuous playback of the contents of the data. Specifically, data remain read out from the shock-proof memory and playback of the contents of the data continues even for a time during which the pickup jumps from a sector to a next sector while kicking across recording tracks on the MD and waiting for disc rotation to meet the next sector.
During the recording mode of operation of the MD player, a pickup sequentially accesses sectors on the MD and records data thereon. In the MD player, compressed data to be recorded are temporarily stored in the shock-proof memory. The compressed data are intermittently read out from the memory before being fed to the pickup and being recorded on the MD thereby. Thus, during the recording mode of operation of the MD player, the feed of data to the pickup is intermittently executed. The absence of data feed to the pickup is synchronized with jump of the pickup from a sector to a next sector. Accordingly, during the absence of data feed, the pickup jumps from a sector to a next sector while kicking across recording tracks on the MD and waiting for disc rotation to meet the next sector.
Optical discs contain a DVD (Digital Video Disc or Digital Versatile Disc). DVD players include shock-proof memories similar in function to those in the MD players. Typical shock-proof memories in the DVD players have a capacity of 16 MB which corresponds to a playback time of about 2 seconds. Advanced shock-proof memories in the DVD players have a capacity of more than 16 MB which corresponds to a playback time of longer than 2 seconds.
Optical discs are of a read only type (a playback only type), a recordable type (a write once type), and a rewritable type. A CD (Compact Disc), a VCD (Video CD), and a DVD are optical discs of the read only type. A CD-R and a DVD-R are optical discs of the recordable type. A CD-RW, a DVD-RAM, and a DVD-RW are optical discs of the rewritable type.
Optical discs of the rewritable type have thin recording films which are reversibly changed between two or more different states in accordance with conditions of laser beams applied thereto. Rewritable optical discs include magneto-optical discs and phase change discs.
In the case of a phase change optical disc, while a recording film is scanned by a laser beam, the recording film is reversibly changed between an amorphous state and a crystalline state by changing conditions of the laser beam in response to a signal to be recorded. Thus, the signal is recorded on the recording film as a pattern of amorphous portions and crystalline portions of the recording film. The signal is reproduced from the phase change optical disc as follows. The surface of an amorphous portion of the disc and the surface of a crystalline portion thereof are different in reflectivity with respect to a laser beam. While the phase change. optical disc is scanned by a laser beam, a change in reflectivity of the disc surface with respect to the laser beam is optically detected so that the signal is reproduced from the disc.
The phase change optical disc is similar to a read only optical disc and a recordable optical disc in the point that signal reproduction is implemented by detecting a change in the disc surface reflectivity with respect to a laser beam. Signal overwriting on the phase change optical disc can be performed by use of only one laser beam when the laser power is modulated between an erasing level Pe and a recording level Pw. Therefore, the structure of a drive device for the phase change optical disc can be simple.
It is conceivable to use a PWM (pulse width modulation) system to record a signal on a rewritable optical disc at a high density. According to the PWM system, the positions of the front and rear edges of every recording mark on the disc correspond to “1” in a digital signal.
In the PWM system, the width of every recording mark represents information. Thus, a desirable shape of the recording mark is free from distortion. Specifically, it is desirable that the shapes of the front and rear halves of the recording mark are symmetrical with each other. During the PWM-based recording of a signal on the disc, the disc is exposed to a laser beam while being rotated and moved relative thereto. In addition, the intensity of the laser beam is changed between strong and weak levels in response to the signal to be recorded. Recording marks are formed on portions of the disc which are exposed to the stronger laser beam. Regarding every recording mark, the heat accumulation effect causes the stronger-beam-application ending point on the disc to be higher in temperature than the stronger-beam-application starting point on the disc. As a result, the rear end of the recording mark is wider than the front end thereof. Thus, the shape of the recording mark is distorted.
Japanese published unexamined patent application 3-185628 discloses a method of reducing distortion in the shape of a recording mark. The method in Japanese application 3-185628 is an overwriting method in which one recording mark is formed by the application of a train of short pulses (narrow pulses) of a laser beam to a disc.
Japanese published unexamined patent application 6-12674 discloses a method of correcting the waveform of a train of electric pulses fed to a laser source. According to the method in Japanese application 6-12674, an input signal repetitively changes between a high level state and a low level state. The input signal being continuously in the high level state corresponds to one recording mark. The input signal being continuously in the high level state is converted into a train of electric short pulses (electric narrow pulses). The first pulse in the train is wider than the second and later pulses therein. The number of the pulses in the train is determined by a desired length of the recording mark. The electric pulse train is fed to the laser source. The electric pulse train is converted by the laser source into a corresponding train of short pulses (narrow pulses) of a laser beam. The laser beam pulse train is applied to a disc. One recording mark is formed on the disc in response to the laser beam pulse train. Since the first pulse in the train is relatively wide, the temperature of the beam-train-application starting point on the disc quickly rises. On the other hand, since the second and later pulses in the train are relatively narrow, the temperature of the beam-train-application ending point on the disc is prevented from excessively rising. Therefore, it is possible to compensate for the heat accumulation effect which would cause distortion of the recording mark.
The shape-distortion reducing technique in Japanese application 6-12674 is less effective as the linear velocity related to the scanning of a disc increases. In the method of Japanese application 6-12674, a train of short pulses (narrow pulses) of a laser beam is applied to the recording film of a disc to form a recording mark thereon. The pulsative laser beam results in decreased energy applied to the recording film of the disc. Accordingly, a required instantaneous power of the laser beam is relatively high. In addition, a required instantaneous power of the laser beam rises as the linear velocity related to the scanning of the disc increases. A high-power laser source is expensive.
In the method of Japanese application 6-12674, the input signal being continuously in the high level state is converted into a train of electric short pulses. It is necessary to use a clock signal in the conversion of the high-level input signal into the electric pulse train. The period of the clock signal is equal to the period of the input signal which is divided by a given integer. As the frequency of the input signal rises, the required frequency of the clock signal increases. An excessively high frequency of the clock signal causes difficulty in circuit designing. Modulation of the laser power at a higher frequency causes greater distortion in the waveform of the laser beam.
In a CAV (constant angular velocity) disc drive system, a disc is rotated at a constant angular speed. In this case, the linear velocity related to the scanning of an outer portion of the disc is higher than that of an inner portion of the disc. According to a proposed method, the length of a recording mark on an inner portion of a disc and the length of that on an outer portion of the disc are set the same to increase the recording density. In the proposed method, the recording frequency at a position on the disc increases as the position is closer to the outer edge of the disc.
In a CLV (constant linear velocity) disc drive system, a disc is rotated at a constant linear speed. A conceivable CLV recording apparatus is able to record signals on discs of different types. The conceivable CLV recording apparatus is required to change the linear velocity and the recording frequency depending on the disc type.
Optimal recording conditions of a disc having a high recording density vary from disc to disc. In addition, the optimal recording conditions depend on the number of times of signal recording on the disc, the ambient temperature, and other factors. According to a conceivable method of detecting optimal recording conditions of a disc, signal recording on the disc is interrupted, and a recording head is moved to a test area of the disc. Then, a test signal is recorded on the test area, and the test signal is reproduced therefrom. The quality of the reproduced test signal is measured. Optimal recording conditions of the disc are detected on the basis of the measurement results. After the optimal recording conditions are detected, the recording of a main information signal on the disc is started. The recording of the main information signal is implemented under the optimal recording conditions. In the conceivable method, the detection of optimal recording conditions takes a long time. Thus, there is a long wait until the recording of the main information signal on the disc is started.
The power of a laser beam depends on the ambient temperature and the aging of a laser source. To maintain accurate signal recording on a disc, it is necessary to compensate for such a variation in the power of the laser beam. In a conceivable method, signal recording on the disc is interrupted, and the power of a laser beam is measured. Optimal drive conditions of a laser source are decided on the basis of the measurement results. In the conceivable method, the decision as to optimal drive conditions of the laser source takes a long time.
A prior-art method of detecting optimal recording conditions of a CD-R has a step of measuring the asymmetry of a reproduced signal. A DVD-R, a DVD-RW, other organic-dye recordable optical discs, other phase change rewritable optical discs, and other recordable and rewritable optical discs having high recording densities are made from various selections of materials in various fabrication methods. Therefore, if the prior-art method is applied to such a high-recording-density disc, the results of the detection of optimal recording conditions are unreliable.
A phase change optical disc has the following problem. As a same signal is repetitively recorded on a same position on the disc at a same timing, the jitter-related quality of a signal reproduced therefrom deteriorates.